This invention relates to semiconductor devices, and more particularly to substrate bias circuits of the type used in semiconductor dynamic memory devices or the like.
Semiconductor memory devices of the MOS dynamic read/write type, as well as other such devices, use substrate pump circuits to generate a negative voltage for substrate bias. These circuits are usually designed as a compromise of several conflicting requirements. The circuits consist of an oscillator driving switches to charge and discharge a capacitor through a diode-type element into the substrate; the frequency of the oscillator and the power level of the capacitor discharge are chosen to maintain the back bias at the proper level in average operating conditions without dissipating an excessive amount of power. But when the power level is chosen to be low the time needed to build up the full bias level after power-on is unduly long.
Leakage of the substrate bias is for the most part caused by impact ionization current. This current peaks when a transistor is pinched off, and is negligible at other times. Transistors are seldom in the pinch off state in an MOS dynamic memory except when output logic states switch, which occurs during an active cycle when RAS and/or CAS are cycled. Thus, the substrate pump is designed for peak load to supply current to compensate for leakage which occurs primarily during active memory cycles, but this results in unnecessary dissipation of power during standby.
It is the principal object of this invention to provide improved substrate pump circuits for semiconductor integrated circuits such as MOS dynamic memory devices. Another object is to provide an improved substrate pump which dissipates a minimum of power, yet builds up the substrate bias rapidly at power-on, and compensates for varying types of operating conditions.